Increased physical activity (PA) is associated with reductions in treatment-related side effects, cancer recurrence and mortality and increased QOL and survival in breast cancer survivors. However, survivors demonstrate persist, significant post-treatment declines in PA, and the majority do not meet PA recommendations. Most existing studies of PA in survivors are intense, on-site multicomponent interventions that provide little insight into what components are, or are not, effective and have limited potential to be brought to scale. The goal of this Career Development Award is to assist Siobhan M. Phillips, PhD, MPH in her long- term goal of becoming an independent investigator in designing and implementing optimized mobile health (mHealth) PA interventions in cancer survivors to improve health and disease outcomes. Dr. Phillips's short- term goals include enhancing or gaining expertise in PA and cancer survivorship, use of mHealth in PA interventions, Multiphase Optimization Strategy (MOST) methodology and dissemination and implementation (D&I) sciences. In collaboration with her multidisciplinary team of mentors and advisors, she has developed a training plan to reach these goals via coursework, seminars, specialized trainings, independent studies, and a mentored research project. The purpose of the proposed research is to use MOST methodology to develop and pilot test a set of mHealth PA intervention components for breast cancer survivors targeted at social cognitive constructs and designed with D&I in mind from the outset. MOST is an innovative, multi-phase framework adapted from engineering that uses highly efficient factorial experiments to evaluate individual, and combined, effects of interventio components to determine which ones can be reduced, eliminated or replaced to improve efficiency. The intervention components to be tested will be chosen in collaboration with survivors, community partners, and experts/clinicians and pilot tested in a 12-week MOST experiment in 250 breast cancer survivors. The proposed experiment examines the feasibility and acceptability of the intervention components and quantifies their preliminary effects on PA and patient reported outcomes at 12 weeks and 24 week follow-up. This information will be used to assemble a PA intervention optimized to maximize PA and minimize cost and participant burden to be tested in a future trial. The proposed study represents the first systematic effort to use MOST to design an optimized, disseminable, mHealth PA intervention for breast cancer survivors and will lead to an improved understanding of how to effectively change survivors' PA. These findings could result in more scalable, effective PA interventions for survivors, and, ultimately, improve health and disease outcomes. Completion of the proposed research and training plans will provide the candidate with the necessary skills, experience, and preliminary data to launch a successful independent research program focused on PA and cancer survivorship.